1. Field of the Invention
This invention relates to a four-stroke reciprocating piston internal combustion engine in which a portion of the exhaust gas is selectively recirculated and added to the cylinder along with fuel and air to create a stratified exhaust gas/air-fuel mixture within the combustion chambers.
2. Description of the Related Art
It is well known that many spark-ignition four-stroke reciprocating piston internal combustion engine designs have a lower efficiency at low/partial load. The loss of efficiency is caused by a number of factors including: losses due to the throttling of the intake mixture, slow combustion due to lower density of the throttled mixture, and excessive heat loss to the combustion chamber walls. In addition to low efficiency at low/partial load, some engines emit unacceptable levels of nitrogen oxides (NOx) and hydrocarbons (HC) during partial-load operation.
Engine designs that use exhaust gas recirculation (EGR) have been developed as one solution to the efficiency and emission problems associated with low/partial load operation. It has been well documented that the use of exhaust gas recirculation in engines provides some reduction in throttling losses and significant reductions in engine NOx emissions because the peak combustion temperature is reduced. It is also known that exhaust gas recirculation can be used with stoichiometric air-fuel mixtures to allow the use of conventional three-way catalysts for effective exhaust emission control.
A number of engine configurations that implement exhaust gas recirculation are known in the art. For example, in U.S. Pat. No. 4,870,941 there is disclosed a basic type of an engine EGR system wherein the engine exhaust manifold is placed in fluid communication with the intake manifold by way of an EGR passage. In this configuration, exhaust gas is recirculated or fed back to the intake manifold through the EGR passage and an EGR valve. Another type of engine EGR system is disclosed in U.S. Pat. No. 5,404,844 wherein a first portion of the exhaust gas in an upper portion of the cylinder is expelled into an exhaust port and a second portion of the exhaust gas from a lower portion of the cylinder is expelled into an intake port for subsequent reintroduction into the cylinder from the intake port during the intake stroke. Yet another type of engine EGR system is disclosed in U.S. Pat. No. 5,005,552 wherein exhaust gas is recirculated into the cylinder by opening the exhaust valve during the intake stroke thereby drawing a backflow of exhaust gas into the cylinder.
Engines that use charge stratification have been developed as another solution to the efficiency and emission problems associated with low/partial load operation. In a typical stratified charge spark ignition engine, the air/fuel mixture is deliberately delivered to the cylinder in a non-homogeneous fashion to produce a small kernel of a relatively rich mixture at the spark plug while the remainder of the air/fuel mixture is relatively lean. As the rich mixture ignites, it in turn ignites the lean mixture in the combustion chamber. In such engines, therefore, the air/fuel mixture can be burned completely even if the air/fuel mixture, as a whole, is lean. Accordingly, fuel economy is improved and regulated gaseous emissions, such as carbon monoxide and NOx, that are contained in the exhaust of the engine can be reduced. However, engines using charge stratification methods typically require special expensive lean NOx trap catalysts for emission control, and therefore have not achieved widespread use.
The charge stratification may take place in different ways within the combustion chamber. The common forms of stratification are axial stratification in which the fuel is concentrated at the top of the combustion chamber and radial stratification in which the fuel is concentrated near the central axis of the combustion chamber. Other forms of stratification are created by causing the intake charge to tumble, that is to say rotate about an axis perpendicular to the cylinder axis. Such motion is not to be confused with swirl in which gases rotate about the axis of the cylinder, the latter normally resulting in radial stratification. When tumble is promoted in the intake charge, stratification can occur in one of two ways. In one way, if the composition of the gases varies across the width of the combustion chamber, a sandwich-like structure with vertical layers is created that is referred to as vertical stratification, the vertical direction being taken as the axis of reciprocation of the piston. In the other form of stratification produced by tumble, which is termed envelope stratification, the tumble produces a horizontal cylinder of one composition enveloped in a second outer layer having a different composition, the two layers tumbling in unison at right angles to the cylinder axis.
In order to produce radial stratification, the gases entering the combustion chamber can be provided as two streams of different composition that are aimed tangentially into the combustion chamber. U.S. Pat. No. 5,765,525 discloses an engine that produces radial stratification by directing one stream to swirl near the cylinder wall while the other stream is directed towards the center of the cylinder.
Envelope stratification can be produced by splitting the intake gases into two streams of different composition with the separation between them extending at right angles to the stem of the intake valve. U.S. Pat. No. 5,915,354 discloses an engine that produces envelope stratification using this method. The lower of the two streams is directed towards the center of the combustion chamber while the upper stream is directed towards the roof of the combustion chamber, the two streams then tumbling together at right angles to the cylinder axis.
Stratified exhaust gas recirculation (EGR) has been developed as yet another solution to the efficiency and emission problems associated with low/partial load operation. U.S. Pat. Nos. 4,393,853, 5,870,993, 5,894,826 and 5,918,577 all represent various forms of stratified exhaust gas recirculation (EGR).
In U.S. Pat. No. 4,393,853, there is disclosed an engine which implements stratified exhaust gas recirculation (EGR) methods. In the engine configuration described in this patent, a swirling fuel-air mixture is introduced through an intake valve into the center of the cylinder near the spark plug, and exhaust gas is directed from another cylinder or the exhaust gas manifold through a tangential port located in the middle of the cylinder wall. This engine configuration creates radial stratification wherein exhaust gas swirls adjacent the cylinder wall and the fuel-air mixture swirls in the center of the cylinder.
U.S. Pat. No. 5,870,993 discloses an engine configuration wherein exhaust gas is recirculated into the cylinder by opening the exhaust valve during the intake stroke thereby drawing a backflow of exhaust gas into the cylinder. A chamfer on the exhaust valve opening creates a swirling flow of recirculated exhaust gas that rotates about an axis parallel to the direction of motion of the reciprocating piston. A fuel-air mixture is also introduced into the cylinder in a manner that creates a tumble flow in the center region of the cylinder. The swirling exhaust gas flow and the tumbling fuel-air flow create stratification in the combustion chamber.
In U.S. Pat. No. 5,894,826, there is disclosed an engine with a first inlet port for introducing air tangentially into the cylinder to produce a generally circular motion in a peripheral region of the cylinder and a second inlet port to introduce air and recirculated exhaust gas, which is provided to the second inlet by way of an EGR passageway, toward the center of the cylinder.
U.S. Pat. No. 5,918,577 discloses another stratified EGR engine wherein exhaust gas is recirculated into the cylinder by opening the exhaust valve during the intake stroke thereby drawing a backflow of exhaust gas into the cylinder. A helical exhaust port creates a swirling flow of recirculated exhaust gas that rotates about an axis parallel to the direction of motion of the reciprocating piston. The exhaust gas remains substantially on the piston surface and along the cylinder wall to create a cup-shaped region. A fuel-air mixture is also introduced into the cylinder in a manner such that the fuel-air mixture swirls in the center of the cylinder. The swirling exhaust gas flow and fuel-air flow create a stratification in the combustion chamber.
While the engine configurations in U.S. Pat. Nos. 4,393,853, 5,870,993, 5,894,826 and 5,918,577 can provide improved engine efficiency and lower emissions, there are certain disadvantages with these prior stratified EGR systems. It is well known that the air-fuel ratio has a significant effect on engine torque and emissions. Particularly, one specific air-fuel ratio, the stoichiometric mixture, is highly significant and corresponds to an air and fuel combination such that if combustion were perfect all of the hydrogen and carbon in the fuel would be converted by the burning process to H.sub.2 O and CO.sub.2. Accordingly, it is preferred that an ignitable mixture be present in the center of the cylinder before the end of the compression stroke when ignition occurs so that emissions can be decreased. As a result, an ideal stratified EGR system should maintain a stratified flow during intake and compression such that an ignitable mixture is present in the center of the cylinder near the spark plug before ignition occurs. One disadvantage with the engine configurations disclosed in U.S. Pat. Nos. 4,393,853, 5,870,993, 5,894,826 and 5,918,577 is that these engine configurations may not maintain a stratified flow during intake and compression which can have negative effects on spark ignition combustion quality and emissions.
For example, the engine configuration in U.S. Pat. No. 4,393,853 provides some level of stratification, but significant mixing of the air-fuel mixture and recirculated exhaust gas is inevitable with this design. As a result, it may be difficult to achieve a near stoichiometric mixture in the center of the cylinder before ignition occurs. Likewise, the engine configuration in U.S. Pat. No. 5,870,993 provides stratification, but mixing of the air-fuel mixture and recirculated exhaust gas is inevitable because of the swirling flow of recirculated exhaust gas and the tumbling flow of the air-fuel mixture. With respect to the engine design in U.S. Pat. No. 5,894,826, significant mixing of air, fuel and exhaust gas will occur in this engine as the fuel is injected through exhaust gas and into air at the peripheral region of the cylinder. Lastly, while the engine configuration in U.S. Pat. No. 5,918,577 provides an improvement in the control of air-fuel mixture and exhaust gas mixing compared to the engines disclosed in U.S. Pat. Nos. 4,393,853, 5,870,993, and 5,894,826, the engine disclosed in U.S. Pat. No. 5,918,577 will also experience some mixing of the air-fuel mixture and recirculated exhaust gas as any recirculated exhaust gas existing on the top of the piston will be mixed with the air-fuel mixture when the piston pushes up on the recirculated exhaust gas and air-fuel mixture during the compression stroke.
Thus, there is a continuing need for a four-stroke reciprocating piston internal combustion engine that optimally implements stratified exhaust gas recirculation (EGR) methods to provide a solution to the efficiency and emission problems associated with low partial load engine operation. More particularly, there is a need for a four-stroke reciprocating piston internal combustion engine that uses stratified exhaust gas recirculation (EGR) methods and can maintain a stratified flow during intake and compression such that an ignitable and fully combustible mixture is present in the center of the cylinder before ignition occurs.